This question arises largely from my desire to "improve" the cases of many of my watches. By improve I mean case them most appropriately for the time they were produced. Lets take one model as an example. The Hamilton 992...at what point is more (or equally) "appropriate" for it to be in a short pendant case as opposed to the tall pendant cases? Anecdotally this seems to happen in the mid 1920's...but recasing muddies all this. When I see "script" lettering on the dial and a high pendant case... it seems right...but is there really a relationship there? But I'm digressing...is there a date for which one begins to think a short pendant is at least reasonable?

Can similar conclusions be drwn for other watch makes?

I typically go by year, probably 1905 and earlier a safe bet is a long pendant. You started to see the no pendant like the Mainliner style cases in the mid 1920's and the short pendant would fall between 1905 (+/- a yr or 2) and 1925 (+/1 a yr or two). I also look at watch ads to see if I am in the ballpark, since a case could sit on the shelf for a few years depending on the cost and turn-around at the jeweler.

I assume the introduction of factory casing had a strong effect as well. This seems to impact both Hamilton (992) and Illinois (Bunn)in particular. I also see an influence in Elgin but not as prounced.

I think up to the Elgin grade 45x series these were not factory cased but Elgin did provide some factory cases as early as the 37x series. By the time you get to the 47x and later series you can say they were factory cased.

With Hamilton others may be a lot more insightful than I am with any of the Hamilton grades.

South Bend did factory case some watches but some were not and I think that would be true for Hampden and since both of those stopped about 1929 the later trend of being marked as "Cased by" or "Cased and timed" was not engraved on the cases. South Bend did have their line of special cases but in ads you see them listed as movement only, that is also true for Hampden. I noticed that much past the mid 1920's then factory casing was pretty much defacto.